DYNAMICAL PREDICTION OF WEST CHINA AUTUMN RAINFALL BY THE NCEP CLIMATE FORECAST SYSTEM

This study investigates the variation and prediction of the west China autumn rainfall(WCAR) and their associated atmospheric circulation features, focusing on the transitional stages of onset and demise of the WCAR.Output from the 45-day hindcast by the National Centers for Environmental Prediction(NCEP) Climate Forecast System version 2(CFSv2) and several observational data sets are used. The onset of WCAR generally occurs at pentad 46 and decays at pentad 56, with heavy rainfall over the northwestern China and moderate rainfall over the south. Before that,southerly wind changes into southeasterly wind, accompanied by a westward expansion and intensification of the western Pacific subtropical high(WPSH), favoring rainfall over west China. On the other hand, during the decay of WCAR, a continental cold high develops and the WPSH weakens and shifts eastward, accompanied by a demise of southwest monsoon flow, leading to decay of rainfall over west China. The CFSv2 generally well captures the variation of WCAR owing to the high skill in capturing the associated atmospheric circulation, despite an overestimation of rainfall. This overestimation occurs at all time leads due to the overestimated low-level southerly wind. The CFSv2 can pinpoint the dates of onset and demise of WCAR at the leads up to 5 days and 40 days, respectively. The lower prediction skill for WCAR onset is due to the unrealistically predicted northerly wind anomaly over the lower branch of the Yangtze River and the underestimated movement of WPSH after lead time of 5 days.